1. Field of the Invention
This invention relates in general to certain new and useful improvements in ophthalmic instruments for aiding in the determination of physical and optical parameters of the cornea of the eye on a real time basis and more particularly, to an improved ophthalmic instrument of the type which is capable of ascertaining thickness, density and surface contour on a real time basis using a television camera and a multiple Scheimpflug corrected slit projector, as well as associated processing.
2. Brief Description of the Related Art
The measurement of optical density of cataracts has been a subject which is becoming more widely addressed in recent times. One of the recent teachings of cataract optical density measurement is set forth in U.S. Pat. No. 4,863,261 to J. Flammer, "Method and Apparatus for Measuring the Extent of Clouding of the Human Eye." The prior art relating to the measurement of optical density is also exemplified in U.S. Pat. No. 4,019,813 for "Optical Apparatus for Obtaining Measurements of Portions of the Eye."
Planning for anterior segment surgery is also a topic which has received increasing attention in recent years, as, for example, in a paper by Lehrman, et al., "Measurement of Anterior Chamber Diameter" and "Biometry of Anterior Segment by Scheimpflug Slit Lamp Photography", reported in Investigative Ophthalmology and Visual Science, Volume 32, No. 3, March 1991, pages 529-532.
The slit lamp is an instrument employed by many optometrists and ophthalmologists for examination of the anterior portion of the eye. Many different versions of the instrument have been produced over the last century, but all of the slit lamps have three major elements in common and which include a projector for providing a collimated image of an optical slit focused on the eye, a bio-microscope or camera for viewing the image and a mechanical support system. In this slit lamp system, the bio-microscope or camera is designed to view the image formed by the projector and is confocal with the projector. The mechanical support system must be elaborate to at least support the subject and the projector and viewing system. Furthermore, the elements must be positioned relative to one another for appropriate examination of the eye.
Pachymetry attachments are available for the slit lamp to be used in clinical environments. These attachments operate so as to displace half the image by a plane parallel glass block interposed in the viewing path. Rotation of the glass block through a calibrated drum changes the optical offset for half of the image and the user places the front edge of the upper image in contact with the opposing edge in the lower half of the image. The reading in the drum is then recorded as the local corneal thickness. This type of pachymeter can accurately define corneal thickness at an unknown location. However, these modified slit lamp devices are slow, expensive and fragile. Moreover, they are quite difficult to operate, and require substantial training on the part of the operator.
One of the most common corneal thickness measurements used in clinical practice today is that of ultrasound. The A-scan ultra sound probe, much the same as with the optical pachymeter, produces a single-point measurement at an unknown locus of the cornea. In addition, this unknown single point is, in reality, the average thickness of an area of several square millimeters in extent. Because the location of the measurement is not repeatable, the data is variable as well.
One of the principal problems of the prior art systems is that the plots which one generated to provide a cornea mapping were not accurate and more importantly, were not repeatable. Thus, the prior art did not provide the ophthalmic surgeon with the data required for planning radial keratotomy or other refractive surgical processes.
Some of the other deficiencies in the prior art techniques used for determining corneal thickness and mapping is hereinafter described in the following Overview Of The Invention. In this Overview Of The Invention, the prior art is, in some cases, also contrasted with the principles of the present invention in order to more fully show the substantial advantages achieved by the present invention. Also, and to some extent, background theory is set forth in order to more fully aid in the understanding of the present invention.